1. Field of the Invention
The present invention relates to a distance sensor for a vehicle for detecting the distance from a vehicle to an object ahead of the vehicle, based on the time from the transmission of an optical signal to the reception of a reflected optical signal by means of transmitting the optical signal ahead of the vehicle and receiving the optical signal reflected from the object, and particularly, a process for determining an abnormality of detection in the distance sensor.
2. Description of the Relevant Art
An auto-cruising system is conventionally well-known, which is designed to allow a vehicle to travel following a target vehicle, by detecting the distance between the vehicle and the target vehicle traveling ahead of the vehicle using a radar device having a laser beam, and controlling the speed of the vehicle, so that the vehicle--vehicle distance is maintained at a predetermined value. In such an auto-cruising system, when a rear portion of the target vehicle is soiled which decreases the magnitude of the reflected signal of the laser beam, or when the target vehicle has raised a large amount of water or snow from a road surface, the state of reception of the reflected signal is deteriorated and as a result, the distance between the vehicle and the target vehicle may not be properly detected, resulting in the possibility that although the target vehicle actually exists, it is mis-determined that the target vehicle does not exist.
There is already proposed in Japanese Patent Application 9-199568, an auto-cruising system in which when the deterioration of the receiving environment is detected based on the magnitude of the signal reflected from the target vehicle and on interrupted or discontinuous supply of data on the distance between the vehicle and the target vehicle, the auto-cruise control is discontinued, or a warning is emitted to a driver.
In the previously proposed system, however, when the reception of the signal is started again after the interruption of the reception of the reflected signal from the target vehicle, the deterioration of the receiving environment is detected. For this reason, it is desirable that the deterioration of the receiving environment can be detected earlier.